Compact shock absorber for an aircraft undercarriage, and an undercarriage including such a shock absorber

ABSTRACT

The invention relates to a shock absorber for an aircraft undercarriage, the shock absorber comprising a main cylinder in which a rod is mounted to slide telescopically, the shock absorber having one or more pneumatic chambers filled with gas, a supply of hydraulic fluid, and throttle means for throttling the hydraulic fluid such that when the shock absorber is shortened, the gas in the pneumatic chamber(s) is compressed and the hydraulic fluid is throttled, in which the pneumatic chamber(s) is/are fully contained in one or more auxiliary cylinders external to the main cylinder, and in each of which a sealed piston is slidably mounted to separate a pneumatic chamber contained in said external auxiliary cylinder from a fluid chamber in fluid flow communication with the main cylinder. According to the invention, the throttle means are disposed at the inlet of a fluid flow connection between the main cylinder and the auxiliary cylinder(s).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of and incorporates byreference in its entirety French Patent Application No. 06 09906 filedon Nov. 13, 2006.

FIELD OF THE INVENTION

The invention relates to a compact shock absorber for an aircraftundercarriage, and to an undercarriage including such a shock absorber.

BACKGROUND OF THE INVENTION

In general, undercarriages include an oleo damper comprising a cylinderin which a rod is mounted to slide telescopically. In directundercarriages, the cylinder constitutes the strut of the undercarriageand the rod carries one or more wheels at its bottom end. Inundercarriages using a hinged rocker beam, the wheels are carried by thehinged rocker beam and the shock absorber is mounted to be hingedfirstly to the rocker beam and secondly to the aircraft or to a strut ofthe undercarriage.

The oleo damper includes at least one pneumatic chamber filled with gasthat is compressed when the shock absorber is shortened. It alsoincludes means for throttling the hydraulic fluid while it is beingshortened, which assumes that there is a supply of hydraulic fluidconnected in series with the chamber for the gas. Damping impacts duringlanding requires the shock absorber to have a certain shortening strokeover which the gas is compressed and the hydraulic fluid throttled. As ageneral rule, this stroke is of the order of a few tens of centimeters.This stroke and the length of the pneumatic chamber and the length ofthe supply of hydraulic fluid determine the empty length of the shockabsorber.

Aircraft are being optimized more and more, and the room available inthe wheel bay for receiving the undercarriage is becoming more and morelimited. In general, the aircraft manufacturer specifies the position(s)of the wheel(s) when the undercarriage is in the extended position andalso the position(s) of the wheel(s) when the undercarriage is in theretracted position. The designer of the undercarriage then attempts todesign an undercarriage having a mechanism that enables the twospecified wheel positions to be complied with and that can accommodate ashock absorber of appropriate length. In certain recent projects thatare particularly optimized, it has not been possible to make use of anoleo damper of conventional design and that is also capable ofsatisfying all of the specified constraints. Other than requesting theaircraft manufacturer to provide more volume in the wheel bay, it wouldtherefore appear necessary to provide a shock absorber that is shorterand more compact, while nevertheless allowing for the strokes that arenormally used.

The situation becomes even more complicated if, as happens with certainhinged rocker beam undercarriages, such as those of helicopters, theshock absorber is also required, in addition, to drive theextension/retraction function for the undercarriage. The retractionstroke is often longer than the shock-absorbing stroke, therebylengthening the shock absorber correspondingly.

Documents GB 2 057 629, EP 0 398 797, FR 819 164, GB 942 804, and EP 1085 233 disclose shock absorbers including at least one externalauxiliary cylinder receiving a pneumatic chamber of the shock absorber.

OBJECT OF THE INVENTION

An object of the invention is to provide an external auxiliary cylindershock absorber that is simple in structure.

BRIEF SUMMARY OF THE INVENTION

The invention provides a shock absorber for an aircraft undercarriage,the shock absorber comprising a main cylinder in which a rod is mountedto slide telescopically, the shock absorber having one or more pneumaticchambers filled with gas, a supply of hydraulic fluid, and throttlemeans for throttling the hydraulic fluid such that when the shockabsorber is shortened, the gas in the pneumatic chamber(s) is compressedand the hydraulic fluid is throttled, in which the pneumatic chamber(s)is/are fully contained in one or more auxiliary cylinders external tothe main cylinder, and in each of which a sealed piston is slidablymounted to separate a pneumatic chamber contained in said externalauxiliary cylinder from a fluid chamber in fluid flow communication withthe main cylinder. According to the invention, the throttle means aredisposed at the inlet of a fluid flow connection between the maincylinder and the auxiliary cylinder(s).

This disposition avoids large pressures developing in the main cylinderdue to throttling not being transmitted over a connection and into theauxiliary cylinder(s). In addition, the throttling means can be designedto be secured to the auxiliary cylinder(s), thereby simplifying thestructure of the main cylinder.

Thus, the pneumatic chamber(s) is/are rejected into one or more externalauxiliary cylinders so that the main cylinder contains hydraulic fluidonly, thereby simplifying construction of the assembly, and offsettingsealing problems to the auxiliary cylinders which are subjected to nostructural mechanical forces other than those coming from the pressuresthat exist within the external auxiliary cylinders. In particular, theexternal auxiliary cylinders are not subjected to deformation as aresult of landing forces. This makes gas-tightness easier to achieve.

According to a particular aspect of the invention, the auxiliarycylinder(s) is/are secured to the main cylinder. Thus, the shockabsorber forms a unitary assembly without it being necessary to providespecific fastenings for the auxiliary cylinder(s) in the wheel bay ofthe aircraft or on the undercarriage.

In a preferred disposition, the auxiliary cylinder(s) is/are fastenedrigidly to the main cylinder, the hydraulic fluid chamber in eachauxiliary cylinder being separated from the hydraulic fluid supply ofthe main cylinder solely by the throttle means disposed at the inlet toeach said auxiliary cylinder. This disposition makes it possible toavoid having recourse to a fluid flow connection.

BRIEF DESCRIPTION OF THE DRAWING

These aspects of the invention and other aspects can be understood morefully in the light of the sole figure of the accompanying drawing, whichis a section view through a shock absorber in a particular embodiment ofthe invention incorporating the extension/retraction function, and shownin the extended position.

DETAILED DESCRIPTION OF THE INVENTION

The invention is illustrated herein for a shock absorber used for ahelicopter main undercarriage of the type that comprises a rocker beamdirectly hinged to the fuselage of the helicopter, the shock absorberdescribed herein being for hinge mounting between the rocker beam and ahinge point on the fuselage.

The shock absorber comprises a main cylinder 1 in which a rod 2 ismounted to slide telescopically. For this purpose, the rod 2 carries asealing piston 3 that slides inside the main cylinder 1 and that definestherein an annular chamber 4 (interrupted by the rod 2), and anuninterrupted chamber 5 filled with hydraulic fluid, forming a supply ofhydraulic fluid. The ends of the main cylinder 1 and of the rod 2 carrylugs 6, 7 for hinge connection of the shock absorber to the fuselage atone end and to the rocker beam of the undercarriage at the other end.

The shock absorber has a first auxiliary cylinder 10 rigidly mounted onthe main cylinder, e.g. as shown by screw-fastening with an interposedsealing gasket, close to the end lug 6 of the main cylinder 1. The firstauxiliary cylinder 10 extends in a direction substantially perpendicularto a longitudinal axis X of the main cylinder 1. A first piston 11 ismounted to slide in sealed manner in the first auxiliary cylinder 10 toseparate the inside thereof into a first pneumatic chamber 12 filledwith gas under pressure and a first fluid chamber 13 in communicationwith the uninterrupted chamber 5 of the main cylinder 1 via a firstthrottle member 14.

The shock absorber has a second auxiliary cylinder 20 rigidly mounted onthe main cylinder, e.g. as shown by screw-fastening with an interposedsealing gasket, in the proximity of the end lug 6 of the main cylinder1. The second auxiliary cylinder 20 extends on the other side of themain cylinder 1 substantially on the same axis as the first auxiliarycylinder 10. A second piston 21 is mounted to slide in sealed mannerinside the second auxiliary cylinder 20 to subdivide the inside thereofinto a second pneumatic chamber 22 filled with gas under pressure, and asecond fluid chamber 23 in communication with the uninterrupted chamber5 of the main cylinder 1 via a second throttle member 24.

The throttle members 14 and 24 are well known to the person skilled inthe art and they are not described in detail herein.

The fluid flow connection between the uninterrupted chamber 5 and theauxiliary cylinders 10 and 20 in this example is as simple as possible,since the auxiliary cylinders are directly in communication with theuninterrupted chamber 5 via the throttle members 14, 24 disposed at theinlet to the auxiliary cylinders, thus making it possible to avoid usingany pipework.

The first pneumatic chamber 12 is inflated to a first pressure, whilethe second pneumatic chamber 22 is inflated to a second pressure that isgreater than the first pressure. Typically, the first pneumatic chamber12 is inflated to 20 bars, while the second pneumatic chamber 22 isinflated to 80 bars.

The operation of the shock absorber during a landing is as follows.Under drive from the rocker beam tending to shorten the shock absorber,the rod 2 is pushed into the main cylinder 1, thereby forcing thehydraulic fluid contained in the uninterrupted chamber 5 to pass throughthe first and second throttle members 14, 24 to fill the first andsecond hydraulic fluid chambers 13, 23, thereby pushing back the firstand second pistons 11, 21 to compress the gas contained in the first andsecond pneumatic chambers 12, 22.

Under normal conditions, the annular chamber 4 is connected via a firstcontrolled valve 30 to a supply of hydraulic fluid or hydraulic tank ofthe aircraft (not shown) typically at a pressure of about 5 bars, suchthat the annular chamber 4 is filled progressively with hydraulic fluidas the rod 2 is pushed in, thereby avoiding any cavitation.

When the helicopter takes off, the gas under pressure in the pneumaticchambers 12, 22 pushes back the hydraulic fluid contained in the fluidchambers 13 and 23 through the throttle members 14 and 24 towards theuninterrupted chamber 5, thereby pushing the rod 2 towards its extendedposition. The hydraulic fluid contained in the annular chamber 4 ispushed back to the pressurized hydraulic fluid supply of the aircraft.

Since the pneumatic chambers are thus offset into external auxiliarycylinders on the sides of the main cylinder, the extended shock absorberthus presents a length L taken between the fastener axes of the end lugs6, 7 that is particularly compact.

In a particular aspect of the invention, the shock absorber is designedto enable the rocker beam to be retracted in order to limit itsaerodynamic drag in flight. For this purpose, controlled shortening ofthe shock absorber is organized as follows. High pressure hydraulicfluid (typically at 206 bars) is injected into the annular chamber 4 viathe first controlled valve 30, thereby tending to retract the rod 2 intothe main cylinder 1. Simultaneously, a second controlled valve 31enables the uninterrupted chamber 5 to be put into communication with aflask 40 such that the hydraulic fluid contained in the uninterruptedchamber 5 can be evacuated into said flask 40. The hydraulic fluidreceived in the flask 40 pushes back a piston 41 slidably mounted in theflask 40, which in turn pushes back the hydraulic fluid present underthe piston 41 towards the supply of the aircraft. It should thus beobserved that the hydraulic fluid from the uninterrupted chamber 5 doesnot become mixed with the hydraulic fluid of the aircraft. When the rod2 comes into abutment against the end wall of the main cylinder 1, thefirst controlled valve 30 is closed such that the hydraulic fluidpresent in the annular chamber 4 is held captive, thereby preventing theshock absorber from expanding.

It should be observed that during this retraction, the pneumaticchambers 12 and 22 are not compressed since their inflation pressuresare higher than the pressure in the flask 40 (substantially the samepressure as the tank, i.e. 5 bars), such that the hydraulic fluid passespreferentially towards the flask 40.

In order to cause the shock absorber to return to its extended position,it suffices to open the first controlled valve 30 to put the annularchamber 4 into communication with the supply of the aircraft, andsimultaneously to open the second controlled valve 31, if closed.Substantially the same pressure then exists on either side of the piston3, and because of the differences in section between the annular chamber4 and the uninterrupted chamber 5, the rod is driven outwards and theshock absorber extends, until it returns to the position shown in FIG.1.

In the invention, the shock absorber is capable of overtravel in theevent of a particularly hard landing. For this purpose, the rod 2includes a hollow orifice opening out to the side of the piston 3 andreceiving an abutment piece 50 in the bore in the rod 2 so as to projectfrom the rod 2. The abutment piece 50 is held to the rod by a pin 51.The abutment piece 50 has an end wall, and a sealing gasket 52 isdisposed between the rod 2 and the abutment piece 50, thus enabling thebore of the rod to be isolated from the uninterrupted chamber 5.

When the shock absorber is used for retracting the undercarriage, theabutment piece 50 comes into abutment against the end wall 53 of themain cylinder 1, thereby defining the retracted position of the shockabsorber. When the shock absorber is used for damping impacts onlanding, the abutment piece 50 normally does not engage the end wall 53of the main cylinder 1. However, during certain hard landings, not onlycan the abutment piece 50 come into contact with the end wall 53, butthe energy that needs to be absorbed can be such that this contact doesnot suffice to stop the shock absorber being shortened. In theinvention, the pin 51 is then designed to shear beyond a predeterminedforce threshold for the purposes firstly of absorbing a fraction of thesurplus landing energy by rupturing the pin 51, and also of allowing therod 2 to perform overtravel in the main cylinder 1, therebyovercompressing the pneumatic chambers 12, 22, also contributing toabsorbing another fraction of the surplus landing energy.

The shearing rupture of the pin propels the abutment piece 50 towardsthe bottom of the bore in the rod 2, such that the abutment piece 50 nolonger co-operates with the sealing gasket 52. The hydraulic fluidcontained in the uninterrupted chamber 5 can then flow (slowly) betweenthe abutment member 52 and the wall of the bore in the rod 2 so as toempty out through a bleed orifice 54 located at the bottom of the rod,thereby contributing to reducing the high pressure that exists in theshock absorber because of the overcompression.

It should be observed that under normal circumstances, the bleed orifice54 runs the risk of putting the annular chamber 4 into communicationwith outside air. To avoid any such air mixing with the hydraulic fluidof the aircraft, a floating piston 55 is disposed in sealed mannerbetween the cylinder 1 and the rod 2 such that the arrival of hydraulicfluid via the first controlled valve 30 takes place under the floatingpiston 55, while air coming from the bleed orifice 54 can only fill thespace that extends between the floating piston 55 and the piston 3 ofthe rod 2. There is thus no risk of the hydraulic fluid of the aircraftbeing polluted by outside air.

The invention is not limited to the above description, but on thecontrary covers any variant coming within the ambit defined by theclaims.

In particular, although the invention is shown herein in associationwith a shock absorber used for a helicopter undercarriage of the typethat includes a rocker beam directly hinged to the fuselage of thehelicopter, the shock absorber shown being hinged between the rockerbeam and a hinge point on the fuselage, the invention is not limited toan undercarriage of that type; for example it can be applied perfectlywell to an undercarriage of the direct type in which the strut forms themain cylinder 1 and the rod 2 carries the wheel(s) directly.

Although the auxiliary cylinders are shown as being fastened directly tothe main cylinder so that the throttle members are disposed at the inletto the auxiliary cylinders, the auxiliary cylinders could be locatedelsewhere. For example, they could be secured to the side of the maincylinder, parallel thereto, the auxiliary cylinders being connected tothe main cylinder via fluid flow connections, e.g. using rigid pipework.It is also possible to fix the auxiliary cylinders in the wheel bay ofthe aircraft, but that would require additional interfaces to beprovided on the aircraft. Under such circumstances, provision should bemade for a fluid flow connection with a flexible hose or a hose capableof accommodating movements of the shock absorber relative to the hose.Under all circumstances, it is preferable for the throttle members to bedisposed at the inlet to the fluid flow connections such that saidconnections and the auxiliary cylinders are not subjected to the highpressures that exist in the uninterrupted chamber 5 while the shockabsorber is being shortened.

Although the present description relates to a shock absorber having twopneumatic chambers, it would naturally be possible to apply theinvention to a shock absorber having a single pneumatic chamber, or onthe contrary having more than two pneumatic chambers. For example, thetwo pneumatic chambers could be placed in series in a single auxiliarycylinder.

1. A shock absorber for an aircraft undercarriage, the shock absorbercomprising a main cylinder in which a rod is mounted to slidetelescopically, the shock absorber having one or more pneumatic chambersfilled with gas, a supply of hydraulic fluid, and throttle means forthrottling the hydraulic fluid such that when the shock absorber isshortened, the gas in the pneumatic chamber(s) is compressed and thehydraulic fluid is throttled, in which the pneumatic chamber(s) is/arefully contained in one or more auxiliary cylinders external to the maincylinder, and in each of which a sealed piston is slidably mounted toseparate a pneumatic chamber contained in said external auxiliarycylinder from a fluid chamber in fluid flow communication with the maincylinder, wherein the throttle means are disposed at the inlet of afluid flow connection between the main cylinder and the auxiliarycylinder(s).
 2. A shock absorber according to claim 1, in which theauxiliary cylinder(s) is/are secured to the main cylinder.
 3. A shockabsorber according to claim 2, in which the auxiliary cylinder(s) is/arefastened rigidly to the main cylinder, the hydraulic fluid chamber ineach auxiliary cylinder being separated from the hydraulic fluid supplyof the main cylinder solely by the throttle means disposed at the inletto each said auxiliary cylinder.
 4. A shock absorber according to claim3, in which the auxiliary cylinder(s) extend(s) in a direction that issubstantially perpendicular to a longitudinal axis of the shockabsorber.
 5. A shock absorber according to claim 1, including controlledmeans for injecting hydraulic fluid into an annular chamber extendingbetween the rod and the main cylinder, and controlled means forsimultaneously exhausting the hydraulic fluid contained in the supply offluid.
 6. A shock absorber according to claim 5, including an associatedflask for receiving the hydraulic fluid coming from the supply of fluid.7. A shock absorber according to claim 1, including abutment meansdefining a retracted position of the rod in the main cylinder.
 8. Ashock absorber according to claim 7, in which the abutment means can beretracted beyond a predetermined force threshold bay so as to allow theshock absorber to perform overtravel, at least in the event of a hardlanding.
 9. An undercarriage including at least one shock absorberaccording to claim 1.